The invention relates to well screens and particularly to well screens having a perforated pipe base of the type commonly used in the production of oil and gas. For many years, spiral wound well screens of the type disclosed in Johnson U.S. Pat. No. 2,046,458 have been used in water wells for permitting the passage of water through the surface of the screen and into a pipe connected at the upper end thereof which carries the water to the surface. Wells for the production of water are generally of much shallower depths than those used for the production of oil and gas and there is usually very little cause to withdraw the well pipe and attached screen once the well is completed. The situation is far different in the production of oil and gas since such wells often extend many miles below the surface of the earth. The greater depth of the wells requires that the well screens and pipes have a much greater resistance to compressive, tensile and torsional loading than is the case with water wells. In actual practice it seems that in more than 50% of the wells drilled that some reason develops for removing a pipe and screen after it has been installed. Since a gravel pack is usually provided around the well screen to provide additional filtering effect, it is obvious that very large loads have to be applied to lift the pipe string out of the gravel pack. Although the well screens used in such situations, such as the "super-weld" screens sold by Johnson Division of UOP Inc., usually have their strength greatly enhanced by being welded at each end to a length of perforated pipe to which they are telescopically attached, it has been found that very large tensile loads can cause the pipe to elongate and break away from the well screen. This type of failure destroys the usefulness of the well screen and opens a large gap between the screen and the underlying pipe perforations through which undesired materials such as sand may enter. The failure may take place even though the pipe usually has a much lower tensile strength per unit of cross-sectional area than does the well screen. The longitudinal rod portions of the well screen are generally made of stainless steel having a generally higher tensile strength that the typical API grade J-55 steel used in the pipe base. However, since the pipe base has a much greater cross-sectional area available to absorb tensile loading than the area of the well screen rods, the pipe base will tend to elongate while the well screen will fail at the juncture of its rods with one of the end support rings.
Failure of a well screen under excess tensile forces can result in displacement of the well screen rods and wrap wire from their usual positions into positions where they can greatly interfere with removal of the pipe from the well or with the attachment of "fishing" tools. Thus, it is preferable to design the well screen so that the integrity of the screen will be preserved and that any failure will take place, not in the screen portion of the assembly, but by a stripping of the threads by which the pipe base of the screen is mounted to the adjoining length of pipe or screen in the pipe string. The thread stripping mode of failure is preferred since it does not interfere with the attachment of "fishing" tools.